1. Field of the Invention
The present invention relates to a threshold voltage model of pocket implant MOSFET and applications thereof.
2. Description of the Prior Art
It is essential to effectively suppress short-channel effect in order to miniaturize Metal Oxide Semiconductor Field-Effect Transistor (herein referred to as “MOSFET” for both n-channel and p-channnel), and therefore, the pocket implant technology is popularly used. This pocket implant generates concentration profile with a peak located at a place deep below a source end and a drain end of a channel by implanting impurities of the type same as substrate impurities with gate used as a mask. Because regions with high impurity concentration formed at the source end and the drain end of the channel in this way absorb electric field induced in the channel direction and, in addition, a threshold voltage rises as gate length decreases, they can suppress reduction of the threshold voltage. Furthermore, because the concentration peak is located at the deep place, it is an advantage that carrier mobility could be less lowered than in the case a substrate with homogeneous high impurity concentration is used.
The pocket implant described above is the technique that could be a key to miniaturization of MOSFET. However, there has been no technique for easily analyzing the concentration profile of the implanted impurities. Consequently, in the pocket implant MOSFET, the dependence of threshold voltage on the drain voltage, channel length, etc. is unable to be accurately predicted and is a bottleneck in designing circuits.
Presently, Berkeley Short IGFET Model, Version-3, 4 (BSIM3, 4) is proposed as a MOSFET model and is served for use in the industry. In this BSIM3, 4, efforts are made to reproduce the measurability by introducing parameters whose physical meanings are undefined so as to apply the model to the pocket implant MOSFET. However, this model basically assumes that impurity concentration of the substrate is homogeneous, and furthermore, assumes that the reverse density of electric charge in an inversion layer is zero. Thus, it is essentially constrained to apply the model to the pocket implant MOSFET which has inhomogeneous impurity concentration distribution in the channel direction.